A glass substrate having excellent thermal resistance, transparency and a low coefficient of linear expansion has been widely used as a substrate for liquid crystal display devices, organic EL display devices, color filters, solar cells, etc. Recently, plastic materials have attracted attention as an alternative to glass substrates for display devices to satisfy requirements, such as reduction in size, thickness and weight, excellent impact resistance, and flexibility.
Examples of materials recently used as such plastic substrates include polyethylene terephthalate (PET), polyether sulfone (PES), polyethylene naphthalate (PEN), polyarylate (PAR), polycarbonate (PC), polyimide (PI), and the like.
However, these materials can cause problems such as deflection of products and disconnection of wiring due to their considerably high coefficients of thermal expansion. In addition, although polyimide resin has a relatively low coefficient of thermal expansion, it is pointed out that the polyimide-based resin is not suitable for substrate materials due to very low transparency, high birefringence and moisture absorption.
To solve these problems, Japanese Patent Publication No. 2004-51960A discloses a transparent composite optical sheet made of an alicyclic epoxy resin containing an epoxy group, a bisphenol A epoxy resin, an acid anhydride curing agent, a catalyst, and glass fiber cloths.
Further, Japanese Patent Publication No. 2005-146258A discloses a transparent composite optical sheet made of an alicyclic epoxy resin containing an ester group, an epoxy resin with a dicyclopentadiene skeleton, an acid anhydride curing agent, and glass fiber cloths, and Japanese Patent Publication No. 2004-233851A discloses a transparent substrate made of a bisphenol A epoxy resin, a bisphenol A novolac epoxy resin, an acid anhydride curing agent, and glass fiber cloths.
However, the composite sheets disclosed in these patents have a glass transition temperature (Tg) within the range of 145° C. to 160° C., which is lower than a process temperature employed during manufacture of the same, and thus have low heat resistance and deteriorated processibility.
In order to increase the glass transition temperature (Tg) of a composite sheet, a method of introducing a bulky aromatic structure into a binder has been proposed. However, in this case, the sheet has reduced flexibility and becomes rigid and thus may not be suitable for flexible substrates. In addition, due to increased viscosity, the sheet is deteriorated in solubility and wettability, and becomes brittle.
Therefore, there is a need for a novel composite sheet that has improved heat resistance without deterioration in flexibility and exhibits excellent wetting properties to glass fibers.